3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) technology is a mobile broadband wireless technology in which transmissions from base stations, which are referred to as enhanced Node Bs (eNBs), to mobile stations, which are referred to as user equipments (UEs), are sent using Orthogonal Frequency Division Multiplexing (OFDM). OFDM splits the transmitted signal into multiple parallel sub-carriers in frequency. The basic unit of transmission in LTE is a Resource Block (RB), which in its most common configuration consists of 12 subcarriers and 7 OFDM symbols as illustrated in FIG. 1. A unit of 1 sub-carrier frequency and 1 OFDM symbol is referred to as a Resource Element (RE), which is also illustrated in FIG. 1. Thus, a RB consists of 84 REs. As illustrated in FIG. 2, in the time domain, LTE downlink transmissions are organized into radio frames of 10 milliseconds (ms) each consisting of ten equally-sized subframes of 1 ms. Further, for normal downlink subframes, each subframe consists of two equally sized slots of 0.5 ms with each slot consisting of seven OFDM symbol periods.
An LTE subframe includes two slots in the time domain and a number of resource block pairs (RB pairs) in the frequency domain. A RB pair is the two RBs in a subframe that are adjacent in time. The number of RB pairs in the frequency domain determines a system bandwidth of the downlink carrier. Currently, system bandwidths supported by LTE correspond to the use of 6, 15, 25, 50, 75, or 100 RB pairs for the bandwidths of 1.4, 3, 5, 10, 15, and 20 Megahertz (MHz), respectively.
The signal transmitted by the eNB in a downlink subframe may be transmitted from multiple antennas, and the signal may be received at a UE that has multiple antennas. The radio channel distorts the transmitted signals from the multiple antenna ports. In order to demodulate any transmissions on the downlink carrier, a UE relies on Reference Symbols (RSs) that are included in the signal transmitted on the downlink carrier. These RSs and their positions in the time-frequency domain are known to the UE and hence can be used to determine channel estimates by measuring the effects of the radio channel on these symbols.
RSs are also used to perform time and frequency synchronization. There are two types of RSs that are present in LTE to facilitate time and frequency synchronization, namely, Primary and Secondary Synchronization Sequences (PSS/SSS) and Common Reference Symbols (CRSs). The Primary and Secondary Synchronization Sequences occur in the sixth and seventh OFDM symbol periods of every fifth subframe for frame structure 1 and are used for initial time and frequency synchronization to the system and cell identification. Therefore, when a UE wakes up from a cold start, the UE first scans for the Primary and Secondary Synchronization Sequences. Once coarse synchronization is achieved, CRSs are used to for fine synchronization to further reduce time and frequency errors. CRSs are also used for mobility measurements, which are also referred to as Radio Resource Management (RRM) measurements.
FIG. 3 illustrates one RB pair in an LTE subframe. As illustrated, the RB pair includes PSS/SSS locations in the sixth and seventh OFDM symbol periods of each RB. In addition, the RB pair includes CRS locations for two antenna ports, namely, port 0 and port 1. For port 0, the CRS locations are within the first and third last OFDM symbol periods of each RB and with a frequency domain spacing of six subcarriers. Further, there is a frequency-domain staggering of three sub-carriers for the CRSs within the third last OFDM symbol of each RB. During each RB pair, there are thus eight CRSs for port 0. Likewise, there are eight CRSs for port 1 in the RB pair arranged as illustrated.
In LTE Release 10 and prior releases of LTE, the CRS for a single antenna port is always present and is spread out as shown over all RBs and subframes whether any data is being sent to UEs in the subframe or not. This ensures very good time and frequency estimation performance but results in large overhead. Additionally, the CRSs create interference in the system that is independent of the data load being carried in a cell. So, even an “empty” subframe or RB generates interference. This interference degrades overall system throughput, especially in heterogeneous network environments where all eNBs do not transmit with the same power. For example, a macro eNB transmitting CRSs at high power can create significant interference to a UE receiving data transmissions from a pico eNB transmitting with low power. Another drawback with the CRSs is that the energy consumption of the eNBs is relatively high since CRSs are always transmitted in every RB of every subframe regardless of the data load being carried in the cell. This results in wasteful energy consumption. Thus, there is a need for systems and methods that reduce overhead and interference resulting from CRS transmissions.